In the electric coffee machines there is often provided the possibility of supplying hot water to a brewing unit for producing coffee and steam for producing hot milk, emulsified milk for producing cappuccino, milk with a dash of coffee, coffee-milk or analogous drinks. To this purpose a single water heater is usually used which, by controlling the heating resistance, can bring the water temperature to a first value (around 90-100° C.) suitable for preparing coffee and to a second value (around 120-140° C.) for producing steam. The steam is supplied through a nozzle to be input directly into a milk container in order to heat and/or emulsify it with air and form a milk foam. Alternatively, the steam is used to feed an emulsifying device wherein, by Venturi effect, the milk is sucked by an outer container, mixed to the steam and, in case, to a determined air flow to produce hot milk or emulsified milk.
In some known machines two distinct water heaters are used, the first one for producing hot water and the second one for producing steam. An example of machines of this kind is described in WO-A-2010/044116.
The two water heaters can be arranged in parallel. In other configurations (US-A-2008/0216665) the water heaters are arranged in series. In each case, the water heaters are constituted by separated members, housed in different areas inside the coffee machine.
US-A-2008/0271608 discloses a flow-through water heater for a coffee machine including a block of heat conductive material embedding a helically developing electrical resistor and two helically arranged water ducts. Water is fed through the first water duct when hot water is required to feed a brewing chamber. Water is alternatively fed through the second water duct if steam is required, instead. The single electrical resistor is controlled to achieve the desired temperature.
DE-85163331 U discloses a double flow-through water heater including a first water duct and a first electric resistor forming a first water heater as well as second water duct and a second electric resistor forming a second water heater. The ducts are connected either in parallel or in sequence. The purpose of the this known device is two obtain a higher water heating rate thanks to the double electric resistor heater arrangement.
Usually, in particular for abiding by the recent laws on energy saving, the used water heaters are without accumulation and usually comprise a duct with small volume, through which the water flows and wherein the water heats instantaneously while flowing by the effect of the heat received from an electric resistance placed in thermal connection with the water duct through a metallic mass wherein the resistance and the water duct are incorporated. In this case the water heater is constituted in substance by a heat exchanger, wherein the water is heated instantaneously, that is while it is requested by the users downstream of the water heater.